1. Field of the Invention
This invention relates to multi-mode missile seeker systems which require the placement of an adjunct sensor in the missile nose, thus blocking the primary radio frequency (RF) aperture and to the specific direction finding (DF) processing techniques implemented to overcome the blocking problem. More specifically, the invention relates to an electronically scanned RF aperture utilizing an off-boresight DF processing technique to overcome effects from the blocking problem in the missile nose presented by the adjunct sensor.
2. Brief Description of the Prior Art
In the prior art, the traditional method of solving the missile DF problem has been to use a gimballed antenna with a null-tracking DF processing technique. A null-tracking gimbal system strives to keep the antenna null pointed at the target and typically uses an inertial unit to obtain the required DF information. This type of system requires a mechanical gimbal that is constantly moving to keep the target in the null and any error in positioning of the null translates directly into DF error. The addition of an adjunct sensor to this type of system distorts the beam due to blockage and diffraction effects and, thus, causes the antenna null position to shift. This null shift results in large DF errors that prevent this type of system from achieving accurate DF performance.
The current art is replacing the prior art antenna with phased arrays.
The addition of an alternate mode of operation to a missile seeker with a primary RF mode of operation is very desirable to diversify the mission capabilities of the missile. The integration of an adjunct sensor into the nose of a missile vehicle is a straightforward method of adding an additional mode of operation to a missile with a primary RF tracking aperture. The adjunct sensor can represent an infrared, laser, millimeter-wave or any other mode of operation sensor which requires mounting in the front of the missile. The integration of the adjunct sensor with the primary RF aperture presents a variety of design challenges in both the mechanical and electrical arenas.
In terms of electrical performance, the RF aperture is required to maintain adequate performance while a significant portion of the aperture is blocked by the adjunct sensor. A primary measure of electrical performance is the ability to provide accurate DF estimation of a target location. This ability is adversely affected by the addition of the adjunct sensor, often to the point where no useful DF information can be obtained from the primary RF aperture.